1. Field of the Invention
This invention relates to circuitry for obtaining electrical power of conventional voltages and frequencies from sources of energy with variable output power.
2. Description of the Prior Art
In the alternate energy field, such as the generation of power through the force of the wind, the force of waves, use of waste energy, use of brake energy regenerated by a moving mass, or light shining on photo cells, there exist solutions to convert the dc electric power generated by the alternate energy source into ac electric power. In particular, in U.S. Pat. No. 4,366,388 Wilkerson shows how to connect the dc power generated by a wind generator to the ac electric power grid via a connecting inverter. This has the advantage, as Wilkerson describes, of synchronizing the generated power to the frequency and voltage of the power supply. It enables the user of such an inverter to make use of all the energy that is being produced as a co-generator of electricity with the power company.
The present invention represents a marked improvement over the inverter circuit invented by Wilkerson, in that it adapts the dc power source in a much better way to the ac electric power grid. Rather than simply connecting the dc power source to the utility circuit once per half cycle of the ac sinusoidal utility voltage, this invention handles the power transfer by many short alternating current pulses that may be varied in duration as well as frequency of occurrence throughout each half cycle of the utility voltage. In addition the alternating current pulses may be transformed to higher voltages or lower voltages as necessary by the difference between the dc input voltage and the momentary output voltage of the ac utility voltage, where momentary should be thought of as a few microseconds in contrast to the ac utility voltage, which has a duration of 8.33 milliseconds for 60 cycle power and 10 milliseconds for 50 cycle power.
This invention also improves on the inductive characteristics of the Wilkerson inverter at low input voltages, when the Wilkerson inverter injects all the power at the leading edge of the ac half cycle, which is equivalent to the behavior of an inductive load connected to the power grid. The present invention injects the power into the power grid throughout the entire half wave in a balanced manner, even at low dc input voltage.
This invention is also related to U.S. Pat. No. 4,445,049 by Steigerwald, which has as its goal the injection of a sinusoidal current into the utility grid that is matched to the power frequency. The Steigerwald invention requires, however, a constant dc input voltage that is at least as high as the highest voltage within the sinusoidal half cycle, which is 155 volts (110 times square-root of 2), if the current is injected into a 110 volt utility circuit. The present invention injects sinusoidal current into the utility grid by choosing the frequency and magnitude of dc current pulses and then filtering those current pulses.
It is an object of the present invention to extract power from a variable voltage dc source and convert the power, such that the power is injected into a utility power source in such a way as to maximize the power output of the dc source.
It is a further object of the present invention to do the power conversion in an energy efficient manner.